The present invention relates to a method for radio communication. The invention is intended in particular for use in radio transmitters, for example of the mobile-telephone system base-station type, for reduction of undesirable intermodulation distortion The invention also relates to a device for such radio communication.
In connection with radio communication, for example in a base station intended for a mobile telephone system, use is often made of an active array antenna for amplification and transmission of the radio signals generated by the base station. According to known art, such an active array antenna usually comprises one or more amplifiers for amplification of the signals in question and for feeding the amplified signals to a corresponding number of antenna elements which may be of, for example, the microstrip type or the dipole type. The antenna also comprises an electric circuit which is intended for distribution of the signal arriving from the base station among the respective amplifiers. The antenna elements can be arranged in a row, for example, or according to another geometrical pattern.
One problem that may arise in connection with previously known base stations with active array antennas is linked to the fact that they are usually intended to be installed in an outdoor environment which is exposed to interference from various transmitters in the vicinity, which in turn generate radio signals across a broad frequency range. When these signals are incident upon the amplifiers of another active antenna, distortion in the form of intermodulation products will be generated in the latter antenna on account of non-linearities in its amplifiers. These intermodulation products will then be retransmitted via the antenna This results in undesirable interference in the transmitter in question, which is of course a disadvantage with this type of antenna.
According to known art, the abovementioned problem can in principle be solved and the intermodulation distortion can be reduced by using an active antenna which comprises amplifiers which have a very high degree of linearity, that is to say at the output stages of each antenna amplifier. A disadvantage of this solution, however, is that it requires relatively high power consumption in the amplifiers concerned, which in turn results in relatively expensive amplifiers.
Another solution to the abovementioned problem is to filter the transmitted signals, to be precise by using band-pass filters which are positioned after each amplifier in the antenna. In this way, Intermodulation products lying outside the transmitted signal-and concerned can be attenuated The intermodulation products which, on the other hand, lie within the transmitted signal-band can be attenuated by arranging insulators at the outputs of the amplifiers. These insulators can then be adapted so that they attenuate the incident, interfering signal which is fed from the antenna elements to the amplifiers, while the transmitted signal going in the direction from the amplifiers to antenna elements is allowed to pass essentially without being attenuated.
A disadvantage of using the abovementioned methods involving filtering and insulation is that these methods increase the cost and the size of the antenna.
In principle, intermodulation products can also be reduced by positioning a given base station as far as possible from interfering radio transmitters in the vicinity. A disadvantage of this solution, however, is that this may conflict with specified requirements for the intended coverage of, for example, a mobile telephone system, and that it may increase the total number of base stations required to cover a given geographical area.
The object of the present invention is to provide an improved method for radio communication, in particular for reduction of intermodulation distortion caused by signals which are incident upon an active array antenna.
The invention relates to a method for reduction of intermodulation distortion in radio communication, in which a signal, which is intended to be transmitted, is fed to an active antenna for transmission in a main lobe. The antenna comprises a plurality of transmitter stages, each of which is supplied with a partial signal and comprises an input stage, an amplifier, an output stage and an antenna element. The method according to the invention is comprises a first phase rotation of each partial signal before the respective amplifier and a second phase rotation of each partial signal after the respective amplifier. Furthermore, the sum of the first phase rotation and the second phase rotation is chosen to have a value which is constant. Moreover, said second phase rotation is chosen so that an intermodulation lobe which is transmitted from the antenna is controlled in a direction which deviates substantially from the direction of said main lobe.
The invention results in an important advantage by virtue of the fact that the main lobe in which the signals of tee antenna according to the invention are transmitted can be made free from intermodulation products caused by incident radio signals from other transmitters in the vicinity. This in turn leads to reduced requirements relating to such intermodulation, for example by virtue of reduced requirements relating to the use of filters and insulators for each amplifier. The invention also leads to reduced requirements with regard to the linearity of each amplifier.